Small bug that creates compatibility issues with other plugins

Hi! This line causes a fatal error in some cases, because the condition is wrong(includes/woocommerce/Woocommerce.php line 112):

if(! WC()->cart && WC()->cart->is_empty()){
    return ;
}

Should be:

if(WC()->cart && WC()->cart->is_empty()){
    return ;
}

This causes some issues with plugins that also mess around with the checkout fields.